Direct fuel injection (DI) systems provide some advantages over port fuel injection systems. For example, direct fuel injection systems may improve cylinder charge cooling so that engine cylinders may operate at higher compression ratios without incurring undesirable engine knock. However, direct fuel injectors may not be able to provide a desired amount of fuel to a cylinder at higher engine speeds and loads because the amount of time a cylinder stroke takes is shortened so that there may not be sufficient time to inject a desired amount of fuel. Consequently, the engine may develop less power than is desired at higher engine speeds and loads. In addition, direct injection systems may be more prone to particulate matter emissions.
In an effort to reduce the particulate matter emissions and fuel dilution in oil, very high pressure direct injection systems have been developed. For example, while nominal direct injection maximum pressures are in the range of 150 bar, the higher pressure DI systems may operate in the range of 250-800 bar.
One issue with such high pressure DI systems is that the dynamic range of the injectors may be limited by the rail pressure. Specifically, when the rail pressure is very high and the engine has to operate at low loads, the injector pulse width may be very small. Under such small pulse width conditions, injector operation may be highly variable. In addition, at very low pulse widths, the injectors may not even open. These conditions can result in large fueling errors.
In one example, the above issue may be at least partly addressed by a method for an engine, comprising: operating a high pressure fuel pump to direct inject fuel at a variable pressure via a first fuel rail, and at a fixed pressure via a second fuel rail, fuel delivery from the pump controlled via an upstream pressure control valve, wherein the second rail is coupled to an inlet of the pump while the first rail is coupled to a pump outlet. In addition, high pressure fuel pump operation may be advantageously used to split the fuel injected into each cylinder between the fixed pressure and variable pressure direct injectors so as to learn the ballistic region of each direct injector.
As an example, a fuel system may be configured with a low pressure lift pump and a high pressure injection pump. The high pressure pump may be a piston pump. An output of the high pressure injection pump may be controlled mechanically, and not electronically, via the use of a magnetic solenoid valve (MSV). At least one check valve and one pressure relief valve (or over-pressure valve) may be coupled between the lift pump and the injection pump. A first fuel rail delivering fuel at variable high pressure to a first group of direct fuel injectors may be coupled to an outlet of the injection pump via a check valve and a pressure relieve valve. Likewise, a second fuel rail delivering fuel at fixed high pressure to a second group of direct fuel injectors may be coupled to an inlet of the injection pump, also via a check valve and a pressure relieve valve. During conditions when the high pressure piston pump is not reciprocating, such as before engine cranking, the check valves, pressure relief valves, and the MSV enables a fixed pressure of the second fuel rail to be raised to lift pump pressure (typically 5 bar (g)). While the pump is reciprocating, the pressure of the second fuel rail delivering fuel to the second group of direct injectors can be raised to the same level as the minimum pressure of the first fuel rail delivering variable pressure fuel to direct injectors (such as at 15 bar). The pressure of the first fuel rail may be further raised and varied by adjusting the pump output via the MSV. Thus, based on engine operating conditions, fuel may be delivered at fixed or variable high pressure to an engine cylinder via direct injection. Further, during selected learning conditions, a ballistic region of the direct injectors coupled to the variable high pressure fuel rail may be learned by applying an injector pulse width in the ballistic region, while operating direct injectors coupled to the fixed high pressure fuel rail with an injector pulse width in the linear region, and observing a change in exhaust air-to-fuel ratio from stoichiometry.
In this way, fixed high pressure direct fuel injection may be provided at fuel pressures that are higher than the pressure provided by a lift pump. More specifically, a high pressure displacement pump can be advantageously used for providing variable high pressure to a first direct injection fuel rail while also providing a fixed high pressure to a second direct injection fuel rail. By raising the default fixed pressure of second direct injection fuel rail to be as high as the minimum pressure of the first direct injection fuel rail, the benefits of high pressure direct injection can be extended over a wider range of operating conditions. For example, smaller amounts/volumes of fuel can be direct injected more accurately via direct injectors coupled to the fixed pressure fuel rail when direct injection of the equivalent amount is limited by the pulse-width or dynamic range of the direct fuel injectors coupled to the variable pressure fuel rail (such as a very high or very low engine speed-load conditions, as well as during engine cold-starts). Further still, fuel may be delivered in a cylinder cycle over a larger number of split fuel injections in the intake and compression stroke by leveraging direct injection from both fuel rails. Overall, fuel injection efficiency is increased and fueling errors are reduced, improving engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.